4832
J. Am. Chem. SOC.1994,116, 4832-4845
7r-Conjugated Poly(pyridine-2,5-diyl), Poly( 2,2’-bipyridine-5,5’-diyl), and Their Alkyl Derivatives. Preparation, Linear Structure, Function as a Ligand to Form Their Transition Metal Complexes, Catalytic Reactions, n-Type Electrically Conducting Properties, Optical Properties, and Alignment on Substrates Takakazu Yamamoto,’J Tsukasa Maruyama,’ Zhen-hua Zhou,t Takayori Ito,? Takashi Fukuda,t Yutaka Yoneda,? Farida Begum,? Tomiki Ikeda,? Shintaro Sasaki,# Hideo Takezoe,s Atsuo Fukuda,s and Kenji Kubotall Contribution from the Research Laboratory of Resources Utilization. Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan, School of Materials Science, Japan Advanced Institute of Science and Technology, 15 Asahidai, Tatsunokuchi, Ishikawa 923- 12, Japan, Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2- 12- 1 0-okayama, Meguro- ku, Tokyo 152, Japan, and Faculty of Engineering, Gunma University, Tenjincho, Kiryu 376, Japan Received November 9, 1993”
Abstract: Dehalogenation polycondensation of corresponding dihalo compounds with a zerovalent nickel complex gives *-conjugated polymers constituted of pyridine units and 2,2’-bipyridine units in high yields. Poly(pyridine-2,5-diyl) (PPy), poly(2,2’-bipyridine-5,5’-diyl) (PBpy), 3-, 4-, and 6-methylated poly(pyridine-2,5-diyl)s (PMePy’s), poly(6hexylpyridine-2,5-diyl) (P6HexPy), poly( 3,3’-dimethyl-2,2’-bipyridine-5,5’-diyl) (P3MeBpy), and poly(6,6’-dihexyl2,2’-bipyridine-5,5’-diyl) (P6HexBpy) are constituted of 42-300 *-conjugated pyridine rings as measured by lightscattering methods. PPy and PBpy have a rigidly linear rodlike structure as revealed by their showing a theoretically limiting pv (degree of depolarization) value of 0.33, and they exhibit a large refractive index increment (An/Ac = 0.59 cm3 g-I) and a large refractive index of = 2.2. Stretching of poly(viny1 alcohol) film containing the PPy or PBpy molecules in its surface region affords a polarizer which shows a dichroic ratio of 45. The PBpy molecules stand upright on a carbon substrate in a PBpy film vacuum deposited on the carbon substrate as revealed by electron diffractometry. On the other hand, PBpy molecules in a film vacuum deposited on a glass substrate are oriented in parallel with the surface of the glass substrate as revealed by analysis of optical second-harmonic generation from the PBpy film, which shows alignment of all the PBpy molecules attached to the glass substrate (1 X 1 cm) in a same direction: coordination of a PBpy molecule to the Si-0-H group is proposed to explain such orientation of the PBpy molecules. PPy exhibits fluorescence with a peak a t 440 nm in a dilute solution (2 X 10-6 M monomer unit), whereas PPy shows an additional excimer-like emission a t 550 nm in a saturated solution ( O S M monomer unit) and PPy and PBpy films emit only the excimer-like emission a t 550 nm. Picosecond time-resolved fluorescence also supports the excimer-like fluorescence. PPy, PBpy, and their alkyl derivatives are electrochemically reduced or n-doped more easily than poly@-phenylene) and poly(thiophene-2,5-diyl),reflecting r-electron-deficient nature of the pyridine ring, and E,-, values of -2.2 through -2.5 V us Ag/Ag+ are observed for the polymers; the n-doping and its reverse reaction (n-undoping) are accompanied by a color change (e.g., yellow in the n-undoped state and blue in the n-doped state for PPy and PBpy). Chemically n-doped PPy and PBpy with sodium naphthalide have electrical conductivities of 1.1 X 10-1 and 1.6 X 10-1 S cm-1, respectively, as measured with compressed powder. On the contrary, PPy, PBpy, and their alkyl derivatives undergo p-doping neither electrochemically nor chemically (e.g., by treatment with AsFs), reflecting the *-deficient nature of the pyridine ring. PBpy and P6HexBpy form complexes with Ru(II), Ni(II), Ni(O), and Fe(II1) species, and cyclic voltammetry of the PBpy-Ru complex reveals electron exchange between the coordinated Ru species, which is considered to occur through the *-conjugation system of the conjugated polymer ligand. The complexes are active for photoevolution of H2 from aqueous media as well as for reduction of C02.
Chemical and physical properties of *-conjugated poly(ary1ene)s are the subject of recent interest,l and a variety of poly(ary1ene)s including poly(ppheny1ene) (PPP), poly(thiophene2,5-diyl) (PTh), and poly(pyrrole-2,5-diyl) have been prepared by various methods. f Research Laboratory of Resources Utilization, Tokyo Institute of Technology. t Japan Advanced Institute of Science and Technology. I Department of Organic and Polymeric Materials, Tokyo Institute of Technology. 1 Gunma University. Abstract published in Advance ACS Abstracts, April 15, 1994.
0002-7863/94/1516-4832$04.50/0
Among the poly(arylene)s, PPP is constituted of the most fundamental aromatic ring and is considered to have a rigidly linear structure. Actually such a linear structure of PPP has been supported by X-ray diffraction analysiszas well as alignment of the PPP molecules perpendicularly to the surface of substrates.3 However, its very low solubility has prevented its structure, chemical properties, and physical properties as a molecule from being revealed. On the other hand, poly(pyridine-2,5-diyl) (PPy), constituted of another fundamental aromatic ring, has received much less 0 1994 American Chemical Society
J. Am. Chem. SOC.,Vol. 116, No. 11. 1994 4833
Preparation and Properties of a-Conjugated Polymers attention4 when compared with PPP, although it also takes a linear structure similiar to that of PPP and may contribute to understanding the chemical and physical properties of such a linear molecules if it is soluble in solvents. In our studies on the preparation of poly(ary1ene)s by dehalogenation polycondensation of dihaloaromatic compounds,3bs4a,5which is based on organonickel chemistry6 and on the chemical and physical properties of poly(arylene)s, we have found that PPy prepared by this method is soluble in some organic and inorganic solvents and PPy as well as its derivatives exhibit interesting chemical and physical properties. nX-Ar-X
+ nM
-
Scheme 1
Hex
PMePy's
catalyst
Wr),
Hex
=
"-'aH1I
(1) PBMePy Mw = 27000 (n = 300) C Ha
We now report the preparation of PPy, poly(2,2'-bipyridine5,5'-diyl) (PBpy), and their derivatives as well as their structure, chemical reactivities, electrical conducting properties, and optical properties. The polymers PBpy and PRBpy (R = Me, n-CaHI3) serve as electrically conducting chelating ligands to form complexes with transition metals.
gives the following a-conjugated polymers' in high yields as shown in Table 1. The following reaction mechanism involving oxidative addition of Pi (X(Ar),X) (i = 1,2, ...) (eq 3), disproportionation (eq 4), and reductive elimination (eq 5) reasonably accounts for the polymerization process.
NI(O)L,
+
X(Af)iX
oxidative addition
L,Ni
/x
(3)
b ) l X
PI
Complex I
Results and Discussion Preparation. The dehalogenation polycondensation using zerovalent nickel complexes, nX-Ar-X
+ nNi(O)L,
-
Wr), + "Nix&,
Complex I
Complex I'
(4)
(2)
(1) (a) Skotheim, T. A., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; Vols. I and 11. (b) Salaneck, W. R.; Clark, D. L.; Samuelsen, E. J. E. Science and Applications of Conducting Polymers; Adam Hilger: New York, 1990. (c) MacDiarmid, A. G.;Heeger, A. J. NRLMemo. Rep. (Proceedings of the Molecular Electron Devices Workshop) 1981,A P A 05816,208. (d) Andre, J.-M.; Delhalle, J.; Bredas, J.-L. Quantum Chemistry Aided Design of Organic Polymers; World Scientific: London, 1991. (e) Kuzmany, H., Mehring, M., Roth, S.,Eds.EIectronicPropertiesof Conjugated Po1ymers;Springer: Berlin, 1987. (fJBradley,D. D.C.;Mori,Y.InEIectronic Properties of Conjugated Polymers IIC Kuzmany, H., Mehring, M., Roth, S.,Eds.; Springer: Berlin, 1989. (g) Collard, D. M.; Fox, M. A. J . Am. Chem. SOC.1991, 113,9414. (h) Guay, J.; Diaz, A,; Wu, R.; Tour, J. M. J . Am. Chem. Soc. 1993, 115, 1869. (i) Tour, J. M.; Lamba, J. J. S.J. Am. Chem. SOC.1993, 115, 4935. 6) Furukawa, Y.; Ohta, H.; Sakamoto, A.; Tasumi, M. Spectrochim. Acta 1991, 47A, 1367. (k) Seki, K. In Optical techniques to characterize polymer systems; Baessler, H., Ed.; Elsevier: Amsterdam, 1989; p 115. (2) (a) Kovacic, P.; Feldman, M. B.; Kovacic, J. P.; Lando, J. B. J . Appl. Polym. Sci. 1968, 12, 1735. (b) Froyer, G.; Maurice, F.; Mercier, J. P.; Riviere, D.; Le Cun, M.; Auvray, P. Polymer 1981, 22,992. (c) Shacklette, L. W.; Chance, R. R.; Ivory, D. M.; Miller, G. G.; Baughman, R. H. Synrh. Met. 1980, I , 307. (d) Sasaki, S.;Yamamoto, T.; Kanbara, T.; Morita, A,; Yamamoto, T. J. Polym. Sci. 1992, 30, 293. (3) (a) Yamamoto, T.; Kanbara, T.; Mori, C. Synfh. Met. 1990,38, 399. (b) Yamamoto, T.; Morita, A.; Miyazaki, Y.; Maruyama, T.; Wakayama, H.; Zhou, Z.-H.; Nakamura, Y .; Kanbara, T.; Sasaki, S.; Kubota, K. Macromolecules 1992, 25, 1214. (4) (a) Yamamoto, T.; Hayashi, Y.; Yamamoto, A. Bull. Chem. SOC.Jpn. 1978, 51, 2091. (b) Yamamoto, T.; Ito, T.; Kubota, K. Chem. Lett. 1988, 153. (c) Yamamoto, T.; Maruyama, T.; Kubota, K. Ibid. 1989, 1951. (d) Yamamoto, T.; Ito, T.; Sanechika. K.; Hishinuma, M. Synth. Met. 1988,25, 103. (e) Schiavon, G.; Zotti, G.; Bontempelli, G.; Lo Coco, F. J . Electroanal. Chem. Interfacial Electrochem. 1988, 242, 131. (f) Yamamoto, T.; Ito, T.; Sanechika, K.; Hishinuma, M. Chem. Ind. (London) 1988, 337. (5) (a) Yamamoto, T.; Yamamoto, A. Chem. Lett. 1977, 353. (b) Yamamoto, T.; Osakada, K.; Wakabayashi, T.; Yamamoto, A. Makromol. Chem., Rapid Commun. 1985, 6, 671. (c) Yamamoto, T.; Morita, A.; Maruyama, T.; Zhou, Z.-H.; Kanbara, T.; Sanechika, K. Polym. J. 1990,22, 187.
Complex I i reductive elimination Complex II
L,Ni
+
X(Ar)l,lX
(5)
Pi+J
We have proposed, on the basis of kinetics, a similar reaction mechanism for the coupling of ArX to give Ar-Ar by using Ni(0) complexes.68 In the mechanism shown above (eqs 3 - 9 , the oxidative addition of aryl halides to the Ni(0) complex* and reductive elimination of R-R from diorganonickel NiRZL, type are well-known, and disproportionation of organo(halo)nickel(II) complex NiR(X)L, to NiRZL, and NiXzL, is also known.9a The disproportionation reaction (eq 4) is considered to be facilitated in polar solvents like DMF,68 and it corresponds to the suitability of the polar solvents like D M F (Table 1) for the present polymerization. (6) (a) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J . Am. C h e m . 3 T 1 9 7 1 , 93, 3350. (b) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J . Am. Chem. SOC. 1971,93,3360. (c) Kohara, T.; Yamamoto,T.; Yamamoto, A. J . Organomet. Chem. 1980,192,265. (d) Komiya, S.;Abe, Y.; Yamamoto, A.; Yamamoto, T. Organometallics 1983,2,1466. ( e ) Tatsumi, K.; Nakamura, A.; Komiya, S.; Yamamoto, T.; Yamamoto, A. J . Am. Chem. SOC.1984, 106, 8181. (f) Zhou, Z.-H.; Yamamoto, T. J . Organomet. Chem. 1991, 414, 119. (g) Yamamoto, T.; Wakabayashi, S.; Osakada, K. J . Organomet. Chem. 1992, 428, 223. (h) Semmelhack, M. F.; Ryono, L. S.J. Am. Chem. Soc. 1975, 97, 3873. (i) Akermark, B.; Johansen, H.; Roos, B.; Wahlgren, U. J . Am. Chem. SOC.1979, 101, 5876. (7) The dehalogenation polycondensation using Mg also gave however, complete transformation of X-Py-X to the intermediate Grignard reagent was somewhat difficult and the polymerization in THF was sometimes accompanied by polymerization of THF.
4834 J . Am. Chem. SOC., Vol. 116, No. 1 1 , 1994
Yamamoto et al.
Table 1. Preparation of PPy and Related Polymers by Dehalogenation Polycondensation of X-Ar-X with a Mixture of Ni(cod)2 and Neutral
Ligand L run 1 2 3 4 5
6 7 8 9
10 11 12 13
14 15 16 17
18 19 20 21 22
monomer (X-Ar-X)’ Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br Br-Py-Br CI-Py-CI c1-Py-c1 c1-P y-CI Br-Bpy-Br Br-Bpy-Br Br-Bpy-Br Br-3MePy-Br Br-4MePy-Br BrdMePy-Br Br-3MeBpy-Br BrdHexPy-Br
ligandb bPY bPY
bPY bPY
PPh3 PPh3 PPh3 Ni(PPh&* Ni(PPhs),’ PPh3 + mah’ cod bPY
PPh3 cod bPY
PPh3 PY bPY bPY bPY bPY bPY
reaction conditions solventc temp, OC time, h DMF 2 60 4 DMF 60 8 DMF 60 DMF 60 16 rt 24 DMF DMF 60 16 HMPA 60 16 60 16 DMF 16 60 DMF 16 60 DMF 16 60 DMF 16 60 DMF 16 60 DMF DMF 16 60 20 70 DMF 28 70 DMF 70 28 DMF 60 16 DMF 60 16 DMF 60 16 DMF 60 16 DMF 48 DMF 60
yield,” % 93 91 100 95 64 95
95 59 66 84
97 99 87 87 95 95 92 80 80
90 85 80
[?If,
mol wtC
dL g-l
A,
nm
2 700 2 400
4 300 3 800 1 200 1900 1300 1600 1 400
0.858
3738 374
1 600 3 300 2 200
2 400 3 200 1 200
3738
630 16 000
1.618 1.018 1.508
12 000 27 000
36 000
3208 3108 34w 3408 321k
BrdHexBpy-Br DMF 60 48 85 21 000 bpy ~. .. X-Py-X = 2,5-dihalopyridine,Br-Bpy-Br = 5,5’-dibromo-2,2’-bipyridine,Br-3MePy-Br = 2,5-dibromo-3-methylpyridine,Br-4MePy-Br = 2,5-dibromo-4-methylpyridine, Br-6RPy-Br = 2,5-dibromo-6-alkylpyridine (R: Me = methyl, Hex = hexyl), Br-3MeBpy-Br = 5,5‘-dibromo-3,3’dimethyl-2,2’-bipyridine, Br-6HexBpy-Br = 5,5’-dibromo-6,6’-dihexyl-2,2’-bipyridine. Bpy = 2,2’-bipyridine, PPh3 = triphenylphosphine, cod = 1,5-cyclooctadiene, mah = maleic anhydride. 1-1.5 mol of Ni(cod)z/mol of the monomer was added except for run 15, where 2 mol of Ni(cod)z/l mol of monomer was added. About an equimolar amount of ligand to Ni(cod)2 was added. DMF = N,N-dimethylformamide, HMPA = hexamethylphosphoric triamide. Yield is calculated on the basis of carbon recovered. e Determined by the light scattering method. f [T] = Inherent viscosity; dL g-1 = 100 mL g-1. 8 In HCOOH. Ni(PPh3)d prepared in situ from NiC12, Zn, and PPh3 was used instead of the mixture of Ni(cod)z and ligand. i IsolatedNi(PPha)4was used instead of the mixtureof Ni(cod)z and ligand. J Maleic anhydridewas added before stopping the polymerization. In C6H.5. 23 a
As the neutral ligand L added to the polymerization system, bpy (runs 1-4 in Table 1) affords PPy’s with higher molecular weights than PPh3 (runs 5-7). The reductive elimination (eq 5) is considered to proceed more easily in cis-NiR2Lz than transNiR2L2, and obtaining higher molecular weight PPy’s in the presence of bpy than in the presence of PPh3 may be related to taking the cis configuration only in the csae of the bpy chelating ligand; on the other hand, divalent Ni(X)(Y)(PR& (where X and Y are alkyl, halide, etc, and PR3 = tertiary phosphine) type complexes usually take trans configuration^.^*^ Since PPy is not soluble in the polymerization solvents, PPy falls out of the solvent as a precipitate. However, as shown in runs 1-4, an increase in the polymerization time seems to lead to an increase in the molecular weight of PPy, suggesting that the polymerization proceeds even in the propagating species in the precipitate. In the case of PPP prepared by an analogous organometallic process, its molecular weight has not been clarified yet due to the lack of solvent for PPP. As for the proceeding of the polymerization even in the precipitate, a similar increase of molecular weight after the formation of precipitate has been reported for poly(3-alkylthio~hene-2,5-diyl).3~ PMePy’s (runs 18-20) are also insoluble in the polymerization solvent, and obtaining high molecular weight PMePy’s indicates such polymerization proceeds more easily in PMePy’s having lower crystallinity due to the methyl substituent. Polymerization of the monomer with a longer alkyl chain, a hexyl group, affords similar results (runs 22and 23). When thepolymer has lower crystallinity, the reactants (monomer, Ni(0) complex, etc.) may be able to approach the propagating polymer species (8) (a) Hidai, M.; Kashiwagi, T.; Ikeuchi, T.; Uchida, Y. J . Organomet. Chem. 1971,30, 279. (b) Fahey, D. R.; Baldwin, 9. A. Znorg. Chim. Acta 1979,36,269. (c) Fahey, D. R.; Mahan, J. E. J . Am. Chem. SOC.1977,99, 2501. (d) Parshall, G. W. J . Am. Chem. SOC.1974, 96, 2360. (9) (a) Yamamoto, T.; Kohara, T.; Yamamoto, A. Bull. Chem. SOC.Jpn. 1981, 54, 1720. (b) Zbid. 1981, 54,2010. (c) Zbid. 1981, 54,2161.
through its amorphous or locally solubilized part, thus making the polymerization able to proceed even in the precipitate. Use of Ni(PPh3), (runs 8 and 9) gives PPy in a lower yield, indicating that blocking of the reaction site of Ni toward X-Ar-X by the four PPh3 ligands retards the polymerization. Thedichloro monomer (runs 12-14) shows polymerization reactivity similar to that of the dibromo monomer. New types of *-conjugated chelating ligand polymers, PBpy, P3MeBpy, and P6HexBpy, are obtained by using 5,5’-dibromo-2,2’-bipyridine and its alkylated compounds as the starting monomers (runs 15, 21, and 23). Characterization. Data from the elemental analysis agree with the structures of the polymers. The minor content of halogen in the polymers suggests that the dihalogenated propagating species such as Pior P i , (eqs 3-5) smoothly undergoes oxidative addition to the Ni(O)L, complex to give species like complex I and/or binuclear nickel complex I11 with a structure Ni(Ar)iNi bridged by the (Ar)i group.IO The main part of the polymer is considered L,Ni(X)(Ar),X complex I
L,(X)Ni(Ar)iNi(X)L, complex I11
+ H+
-
H(Ar),X
+ H+
-
H(Ar),H
(6) (7)
to be obtained from such chemical species, which will give H-terminated polymers during the workup of the polymers involving treatment of the crude polymer with protic solvents and reagents. PPy, PBpy, PMePy’s, and P3MeBpy are soluble in formic acid and concentrated H2S04. Evaporation of formic acid from the (10) A 1:2 molar reaction between 4,4’-dibromobiphenyl and a zerovalent
nickelcomplex (a mixture of Ni(cod)2 and 2PEt3) actually led to the formation of a complex of type complex 111, (PEt3)2Ni(Br)-Q,&-C6H4-Ni(Br)(PEt3), which was isolated and characterized by elemental analysis and X-ray crystalography (Kim, Y.-J.; Sato, R.; Maruyama, T.; Osakada, K.; Yamamoto, T. J . Chem. SOC.,Dalton Trans., in press).
Preparation and Properties of
7-
Conjugated Polymers
J. Am. Chem. Soc., Vol. 116, No. 11, 1994 4835
formic acid solutions of these polymers gives the original polymer, as proved by IR spectrometry, and thus indicates that formic acid essentially works as a solvent. On the other hand, these polymers seem to be dissolved in concentrated H2SO4 as salts with H2SO4. P6HexPy and P6HexBpy are soluble in most organic solvents including CHC13, THF, benzene, formic acid, and hexamethylphosphoric triamide (solubility = 200-300 mg cm-3 for these solvents). Enhancement of solubilities of poly(ary1ene)s by introducing an alkyl substituent and without loosing the essential *-conjugation along the polymer chain has been reported.’ Determination of the molecular weight of P6HexPy by light scattering by using two kinds of solvents, formic acid and chloroform, gives essentially the same value (37 000 (DP = 230) in formic acid and 36 000 (DP = 220) in chloroform, respectively). The light scattering method also reveals that P6HexPy has a monodispersed relatively narrow molecular weight distribution with a M,IM, value of about 2 ( M z = Z-average molecular weight; M , = weight-average molecular weight). In cases of PPy, PMePy’s, and P6HexPy, they contain both the head-to-tail H-T and head-to-head H-H units. As for R
(b) PBMeBPy
1
R
1 H-1
H-H
P3MePy and P6HexPy, the H-T to H-H ratios are determined by comparing their lH-NMR spectra with those of P3MeBpy (Figure 1)128and P6HexBpy, and the ratiosI3 thus obtained are
for P3MePy H-T/H-H = 65/35
(8)
for P6HexPy H-T/H-H = 55/45
(9)
In the case of PPy, it was difficult to determine the H-T/H-H ratio from IH-NMR; however, the above data (eqs 8 and 9) suggest that PPy is also constituted of H-T and H-H units. The considerably lower coordinating ability of PPy to form metal complexes than PBpy (vide infra) suggests a relatively low content of the H-H unit. A mixed coupling reaction between 2-bromopyridine and 3-bromopyridine by using an Ni(0) complex affords the following mixture 60°C. DMF
12.3 NIL,
:
60.4
:
27.3
: 1 : 1 : 1 mixture of Nl(cod),, PPh,
and 1,5-cyclooctadiene
and the preferential formation of 2,3’-bipyridine is consistent (11) (a) Yamamoto, T.; Sanechika, K. Chem. Ind. (London) 1982, 301. (b) Yamamoto, T.: Sanechika, K.; Yamamoto. A. US Pat. 4521589, 1985. (c) Elsenbaumer, R. S.;Jen, K. Y. J.; Oboodi, R. Synth. Met. 1986,15, 169. (d) Riihe, J.; Ezquerra, T.; Wegner, G. Makromol. Chem., Rapid Commun. 1989, 10, 103. (e) Kaeriyama, K.; Sato, M. Makromol. Chem., Rapid Commun. 1989, 10, 171. (12) (a) Maruyama, T.; Zhou, Z.-H.; Kubota, K.; Yamamoto, T. Chem. Lett. 1992,643. (b) Yamamoto, T.; Maruyama, T.; Ikeda, T.; Sisido, M. J . Chem. SOC.,Chem. Commun. 1990,1306. (c) Yamamoto, T.; Zhou, Z.-H.; Kanbara, T.; Maruyama, T. Chem. Leu. 1990, 223. (d) Yamamoto, T.; Yoneda, Y.; Maruyama, T. J . Chem. Soc., Chem. Commun. 1992, 1652. (1 3) More detailed analysis of the IH-NMRdata based on triad and tetrad units may become possible by analyzing the complex peak pattern of the pyridine protons (e&, Figure 1); such analysis has been carried out for poly(3-alkylthiophene-2,5-diyl):(a) Souto, Maior, R. M.;Hinkelmann,K.; Eckert, H.; Wudl, F. Macromolecules 1990,23,1268. (b) McCullough, R. D.; Lowe, R. D. J . Chem. Soc., Chem. Commun. 1992, 70. (c) Mao, H.; Xu, Bai.; Holdcroft, S. Macromolecules 1993, 26, 1163.
1
1
1
1
1
1
1
1
1
1
1 2 1 1 1 0 9
1
1
8
7
6
5
4
3
2
1
0
5 1 PPm Figure 1. 1H-NMR spectra of (a) P3MePy and (b) P3MeBpy in CF3COOD at room temperature.
with the assumption that PPy mainly consists of the H-T unit. In eq 10, 3,3’-bipyridine is formed in a higher ratio than 2,2’bipyridine, which is consistent with the previously reported higher reactivity of the C-Br bond at the 3-position than that at the 2-position.14 I R spectra of the polymers are reasonable for their structures. IR spectra of PBpy, P3MeBpy, and P6HexBpy resemble those of PPy, P3MePy, and P6HexPy respectively, although some differences are observed in the regions of 6(C-H) out-of-plane vibration, 6(CH3) vibration, and 6(CH2) vibration between the polypyridine type polymers and polybipyridine type polymers. Detailed N M R and IR data for the polymers are given in the supplementary material. All the polymers have high thermal stability. TGA indicates that no thermal degradation of PPy and PBpy takes place below 270 O C and their residual weight at 900 OC is about 75%. Even P6HexPy having the alkyl substituent has high thermal stability, showing a residual weight of 95 and 47% at 450 and 900 OC, respectively. PPy and PBpy give rise to a relatively sharp (half-width = 49 nm or 3500 cm-1 (0.44 eV)) single *--A* absorption peak a t 373 f 1 nm or 3.32 f 0.01 eV in formic acid,4bJ2b and the peak position is essentially unvaried with the change of the molecular weight (Mw)of PPy in a range of M , = 1200-3800. The *--a* transition energy of PPy is comparable to that of PPP (A, = ca. 380 nm or 3.26 eV measured in the solid state) prepared by the organometallic Band gaps of PPy and PBpy estimated from the edge (430 nm) of the absorption spectrum are 2.88 eV. Introduction of the alkyl substituent causes a shift of the U-T* absorption to shorter wavelength by 30-60 nm (Table 1). A similar hypsochromic effect of the substituent (presumably by disturbing the coplanarity of the *-conjugated polymers) has been reported for poly(thiophene-2,5-diy1).3bJs Among PMePy’s, P6MePy exhibits the lowest hypsochromic effect probably due (14) (a) Tiecco, M.;Testaferri, L.;Tingoni, M.; Chianelli, D.;Montanucci, M. Synthesis 1984, 736. (b) Parham, W. E.; Piccilli, R. M. J . Org.Chem. 1977, 42, 257. (15) Yoshino, K.; Onoda, M.; Love, P.; Sugimoto, R. Jpn. J . Appl. Phys. 1987, 26, L2046. (b) Hotta, S. Synth. Met. 1987, 22, 103.
4836 J. Am. Chem. Soc., Vol. 116. No. 11, 1994
Yamamoto et al.
Scheme 2
d & p & m -
.........
.........
0.27 in formic acid. P3MePy film has an value of 1.70. A copolymer constituted of pyridine-2,5-diyl units and pyridine2,6-diyl units prepared by the reaction shown in eq 1land having
a 2 7
A
a3
pv 0.33 a1 >> a2, a3
20
to less steric repulsion between the Me groups in P6MePy than in P3MePy and P4MePy. A CPK molecular model of form b of P6MePy indicates minor steric repulsion between the monomer units. Me
......... or
.........
99
.........
PBMOPy
/ \ / \ .........
P3MoPy
.........
-N
Me
Me
Me
-N
Form b
Form e
Me
Me
Me
d-q &C
.........
/ \
BN\.........or
.........
me
Me
.........
......... or
.........
MO
.........
P4MoPy
Linear Structure and Alignment of Molecules. Irradiation with a polarized Ar+ laser (488 nm) of the formic acid solution of PPy and PBpy reveals that the polymers give a very large degree of depolarization of the polarized laser with a pv valueI6 of 0.33, which agrees with the theoretical limiting value of pv = l / 3 calculated for. a molecule in which the polarizability along a rod axis direction (a1)is very large whereas the polarizability along the other two directions (a1and a3) is negligible compared with a1.l6 The observed pv value is, to our knowledge, the largest value reported, and the results strongly suggest that PPy and PBpy take a rodlike rigid linear structure having a u-conjugation system with mobile electrons along the polymer chain (presumably the direction of a1). Due to the presence of mobile electrons along the polymer chain, PPy exhibits a very large refractive index increment of An/Ac = OS9 cm3 g-I in formic acid, which has nD of 1.37. The An/Ac value is much larger than those of usual non-*-conjugated polymers (An/Ac = 0.1-0.2 cm3 g-l) and fairly large compared to that of poly(3-hexylthiophene-2,5diyl) (An/Ac = 0.35 cm3 g-1),3bhaving alkyl substituents. Film of PPy exhibits a very large refractive index of = 2.2, a value somewhat lower than that of diamond n D = 2.4. In contrast to PPy and PBpy, PMePy's, P6HexPy, and P6HexBpy with large molecular weights give pv values of almost 0, indicating that these molecules take essentially a random-coil like structure as a whole molecule probably due to steric repulsion between the alkyl substituents, twisting out of the u-conjugated plane, and/or bowing of the long molecular chain.16c That P3MePy takes a locally linear structure has been confirmed by using the low molecular weight (M,= 770, n = ca. 9) part of P3MePy extracted by hot CHCl3, which shows the pv value of
:
1
a relatively low molecular weight (M,= 950, number of pyridine unit = ca. 12) exhibits the u-r* absorption peak a t shorter wavelength (A, = 353 nm in formic acid) and gives a lower An/Ac value of 0.34 cm3 g-l and a smaller pv value of 0.15 than compared with PPy. Breaking the u-conjugation and bending the molecule at the pyridine-2,6-diyl unit account for the results. A similar effect of the incorporation of nonconjugated units into the r-conjugated polymer to cause a similar hypsochromic shift of the ?r-u* absorption band has been reported for copolymers of ?r-conjugated thiophene-2,5-diyl and non-u-conjugated thiophene-2,4-diyl units." Because of the linear structure of PPy and PBpy, they are aligned in stretched film as well as on substrates. As shown in Figure 2a, painting of poly(viny1 alcohol) (PVA) film with a formic acid solution of PPy and drying the film gives the PVA film containing the linear PPy molecules in the surface region. Stretching the film is expected to lead to the alignment of the PpY molecules as shown in Figure 2b, and actually, irradiation of the film with polarized light (Figure 2c) reveals a strong dichroism (or dependence of the absorbance on the angle 6 in Figure 2c) as shown in Figures 3 and 4. In Figures 2 and 4, the stretching ratio R, and the dichroic ratio Rd are defined as R, = R I I / R , (Figure 2a,b) R, = AII/A, = Ao/A,,
(12)
(Figures 2c and 3) (13)
A = absorbancc
The results depicted in Figures 3 and 4, which show such an unusually large dichroic ratio Rd of 4 0 4 5 , support the alignment of the PPy molecules on the stretched PVA film and reveal that larger dichroic ratios are obtained with PPy's having larger molecular weights. PBpy (with 42 pyridine rings (Scheme 1)) gives, in the PVA film, an Rd value almost the same as that observed with PPy with n value of 49 (Figure 4). Because of the strong dichroism, the stretched film serves as a polarizing film; for example, two overlapped PVA-PPy films with a 90° angle between the stretching directions of the two films cut light at about 400 nm almost completely whereas light at 400 nm passes through two similarly overlapped films with a Oo angle between the stretching directions of the two films. The Rd value for the PVA-PPy film is stable and unvaried even after leaving the film a t room temperature for 4 years. Casting a homogeneous formic acid solution containing PVA and PPy gives a free-standing film containing PPy molecules in bulk PVA, and stretching of the film also affords a film showing dichroism. However, the Rd value of this stretched film is considerably lower (Rd = 2.5, 3.2, and 4.4 at R, = 2.0, 5.0, and 10.2, respectively), and the difference in the Rd values between the two types of stretched films may be attributed to an excellent special anchoring effect of the surface of the PVA film for alignment of the linear PPy molecules and/or a difference in the nature of the disorder and ordering between the PPy molecules (16) (a) Kubota,K.;Urabe,H.;Tominaga,Y.;Fujime,S.Macromolecules in the surface region (2D disorder and ordering, Figure 2a,b) and 1984, 17, 2096. (b) Kubota, K.; Chu, B. Biopolymers 1983, 22, 1461. (c) those in the bulk (3D disorder and ordering). Zero, K.; Aharoni, S. M. Macromolecules 1987, 20, 1957. (d) Saito, N. Figure 5 (top) shows the electron difraction (ED) pattern of Kobunshi Butsurigaku (Polymer Physics); Shokabo: Tokyo, 1976; pp 181, 210. p.= 362/(5+462),where62= ( ( ~ ~ - L I Z ) ~ + ( ~ Z - L Y( a~ ~) -~~+r 1 ) ~ ) / 2 ( a 1 a PBpy film (thickness = 60 nm) formed on a carbon substrate
+ a2 + a#; under the conditions of a,>> a2 and (~3,132and pv become 1 and
l/3, respectively. Poly( 1.4-phenyleneterephthalimine)or Kevlar having a rather rigid linear structure exhibits p. values of 0.175 and 0.11 at M , values of 4500 and 35 000, respective1y.l"
(17) (a) Yamamoto, T.; Sanechika, K.; Yamamoto, A. Bull. Chem. SOC. Jpn. 1983, 56, 1497. (b) Sanechika, K.; Yamamoto, T.; Yamamoto, A. J . Polym. Sci., Polym. Lett. Ed. 1982, 20, 365.
Prepararion and Properries OJ r-Conjugared Polymers
J . Am. Chem. So c., Yo/. 116, No. 1 1 .
1994 4837
f3=90" A l 1.1
ibl
Rs = RII I R l Figure 2. Stretching of PVA film containing linear PPy molecules in its surface region, definition of b!, (Figure 2a.b). and irradiatk I of polarized light on the stretched film (Figure 2c). The four 0 marks are made at the corners of a square (Figure 2a.) to determine the R, value (F igurc 2b). The film was obtained by painting the PVA film with a formic acid solution of PPy, drying the film under v acuum, and stretching the P VA film.
300
so0
400
Wavelenlllll I m
500
Figure 3. Absorption of polarized light by the stretched PVA-PPy film.
Rd
45
-
40 55
-
30
-
25
-
0 : for n=25 n=49
a: for
Rs Figure 4. Relationship between Ra and R, obtained with PPy with n (degree of polymerization) of 25 and 49. PBpy with 42 pyridine rings gives almost the same results obtained with PPy with the n value of 49.
by vacuum deposition. The ED pattern of the vacuum-deposited thin film of PPP' is also shown in Figure 5 for comparison. The ED patterns shown in Figure 5 are taken by using an electron beam (60 keV corresponding to a wavelength of 0.0050 nm) irradiated perpendicularly to the carbon substrate. The ED pattern of the PPP film has been reasonably accounted for by assuming that the PPP molecules stand upright as depicted in Figure 6b to form an orthorhombic packing;' the orthorhombic
Figure 5. FI :ctron diffraclion pattern of (top) Pup) film and (botiom) PPP film (from rcf 3) vacuum deposited on a carbon sut st rate. packing, c,f PPP has been extensively studied by PO diffract.ometry.2 The o'bservation of analogous regular spots in the of the vacuum-deposited PBpy suggests that the PBp also stand upright ov the carbon substrate (Figure an orthorhombic packing, although the observation numlxr of the regul.ar diffraction spots than that ot
wder X-ray ED pattern ~ymolecules 6a) to form of a smaller iserved with
4838 J. Am
,
Chem. Soc.. Vo.!. 116. No. I I , 1994
Yamamoio ei a/. I
I
I:
N
1
h
I I
I
I I
!
(a)
\
(b)
substrate
PBPY Figure 6. Per, mdicular orientation of (a) Pi0 :py and (b) PPP. The detailed packing structure in the ab plane and twisting of the aromatic rings are neglected for I ;implification.
the PPP filn PBpyfilmprf of PBpy tha irregularities bonding and membered ai of 'PBpy see! detailed anal As for PP: of 1500-200 about 20 and vacuum.l9 I PBpy molec considered t( nm. Theref considered t( about 6-8 la regular cleai directly cont PBpy moleci to the surfac The lateri vacuum-dep, chain) are 0. are somewhi = 0.78 nm, disappeared degradation the electron depositional: only much 11 ofPPy. Thi ~
indicates a lower degree of crystallinityI8 of the :sumablyduetoa lessregularai Idlesslinearstructure m that of PPP. For examp le, PBpy may have due to the presenceof s-cis ands -trans stereoisomeric nonlinearity due to the presei ice of N in the six.omatic ring. As a polymerized powder, the sample n s to have a monoclinic cryst, 11 structure, whose .ysis will be reported elsewhere. P, it is reported that PPP's with molecular weights 0, corresponding to a degree of polymerization of a chain length of 8-10 nm, can t E vaporized under f PBpy has volatileness similar ti > t h a t of PPP, the ules in the vacuum-deposited tl iin film are also ,have about 20 pyridine rings and I a length of 8-10 ore, the thickness (60 nm) of t he PBpy film is ,correspond to an accumulation, $3 n the average, of yers of the PBpy molecules, and cil xervation of the ' spots suggests that not only the ifirst PBpy layer acted with the surface of the substr 'ate but also the des in the upper layers areoriented lqerpendicularly e of the substrate. tI dimensions of the orthorhombic ~ m i cell t of the Jsited PBpy(thecaxis is thedirectior,i ofthe polymer 81 nm ( a ) and 0.57 nm ( b ) ,respective:l'y. Thevalues it larger than the ab parameters of PF'P and PTh ( a b = 0.56 nm).2.3.M The spots shown! in Figure 5a after long irradiation by the electron bea m, indicating oftheoriented structure by thelong irrzidiation with beam. Use of PPy, instead of PBpy, in the vacuum iogivesa similar film; however, its EDpattern exhibits :ss sharp dim spots due to the less regular structure :vacuum deposition of PPP to a gold sulmtrate also I
(18)Thent the degree of c , the clear swts dewnds on delicate emerimental conditions for Vhc vacuum depasition'(e.g ., kmperature of subst&). (19) Brown ,C. E.; Kavacic, P.; Wilkic, C. A,; Cody, R. B., lr.; K.insinger. J. A. J . Polym Sci,, Polym. Lell. Ed. 1985. 23, 45.3 and mass Spe(:tr,9 in this reference. ). Z.; Lee, K.-B.; Moon,Y. B.: Kobayashi. M.; H c e w ' . A. J.; ~romolecules1985, 18, 1972. (b) Bruckner. S.;Parzio, W. ,em. 1988, 189.961. ~I
~~~
leads to a similar orientation of the PPP molecules perpendicular to the surface of the substrate.' However, in the case of PBpy, the film vacuum-deposited on the gold substrate does not give rise to a clear ED spot. Interaction of the PBpy molecule with the gold substrate through partial coordination of PBpy to gold may be the reason for the difficulty in making the oriented structure. Vacuumdepositionof PBpyona glasssubstratecausesanother type of alignment of the PBpy molecules on the surface of the substrate as found by using the S H G (optical second-harmonic generation) technique." Although PPP and PPy films vacuum deposited on the glass substrate are SHG inactive, similarly vacuum-deposited PBpy film is SHG active, indicating that the PBpy molecules take a special alignment on theglasssubstrate toafford adomain which has asymmetry in inversion. The intensity of the SH light ( I S H ) from the glass vacuum-deposited PBpy film is much stronger than the background S H light from the glass substrate and proportional to the second power ofthe intensity ofthe irradiated laser (fc),IsH = rez. A detailed study of S H G has been carried out by irradiation with a laser (1064 nm) and detection of the SH light in P-P (irradiation light = P polarized light; detected SH light = P component of the S H light), S-S, S-P, and P-S combinations as shown in Figure 7, and the obtained results are shown in Figure 8. The SH intensity is predominantly observed in the P-Pcombination, and the SH intensities in theother three combinationsaresmaller by 1 order of magnitude, implying that there exists a main nonlinear susceptibility component perpendicular to the surface plane. As is clear from Figure 8a, one of the interesting features of the S H G of the PBpy is the anisotropy of the S H G concerning the rotation around the Z axis: the point symmetry is C,with only one mirror in the X-Z plane. This feature strongly suggests that all the PBpy molecules align in a certain direction and the molecule plane tilts from the surface normal. However, the (21) (a) Shen, Y. R. Annu. Reo. Phys. Chcm. 1989, 40, 327. (b) Chcn. W.; Feller, M. B.; Shen, Y . R. Phys. Reo. Lcli. 1989,63,2665. (c) Shen, Y . R. " W e 1989, 337, 519. (d) Fukuda, T.; Kanbara, T.: Yamamoto, T.; Fujioka, T.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Mol. Ciysl. Liq. Cryst. 1993, 226,207.
Preparation and Properties of r-Conjugated Polymers
\0:1064
nm
- ,_
b,
Film
-8
Analyzer 't
&,
2ti~532nm
Figure 7. Geometry of the experimental system for measuring SHG. Polarized (Sand P) light is irradiated t o the PBpy filmvacuum deposited on theglasssubstratewhichcan be rotaf.edaround theZaxis. Coordinates (XO, YO,20)are those of the laboratrxy frame.
270
270
278 270 Figure 8. SHG frcm the PBpy vacuum deposited on the glass substrate for (a) P-P, (b) S--P, (c) P-S, and (d) S-S combinations. Observed data and best theoreti cal fits are shown by 0 and-, respectively. The relative scale of theSHG intensity isshownon the radical line. Theangleindicates the rotation aro,und the Z axis. The directions giving the minimum SHG and maximum. S H G for the P-P combination are taken as Oo and 180°, respectively.
vacuum-deposited PBpy film does not show observable dichroism in its UV- -visiblespectrum,while alignment of the PBpy molecules in the s'iretched PVA film is clearly observed (Figure 2, vide anre). Therefore, most of the PBpy molecules in the vacuumdeposi.ted PBpy film are not oriented to the certain direction paral'lel to the surface22 and the SH light is mainly generated fron.1the interface between the glass substrate and its neighboring lay er of PBpy. Fitting theoretical equations (cf. the supplementary material) tso the observed data, with a least-squares method, gives relative values of the six tensor elements for S H G (WX, XUW,XUZZ, X X Z X , X W Z , and XZZZ); xzzz is the dominant tensor element, and other five elements are 20-50 times smaller than XZZZ. The (22) The PBpy molecules in the upper layer above the layer neighboring the glass substrate may be aligned perpendicularly to the surface of the substrate. However, ED analysis of the PBpy on the glass substrate has not been possible because of the luckof the thin glass substrate which is transparent to the electron beam and suitable to the ED analysis.
J. Am. Chem. SOC.,Vol. 116, No. 11, 1994 4839 theoretical best-fitted S H G profiles shown in Figure 8 (solid line, = 6, xuw= 3, at relative values of the tensor elements of xuzz = 9, uzx = 3, x m = 4, and xzzz = 160) agree with the observed SHG profiles (0 marks), supporting the assumption of the C, symmetry. If the PBpy molecules are oriented perpendicularly to the surface of the glass substrate as in the case of the PBpy film vacuum deposited on the carbon substrate, such S H G profiles as those shown in Figure 8 would not be expected. On the contrary, if one assumes an orientation of the PBpy molecules along the surface of the Si02 glass as shown in Figure 9a, the molecules are expected to have the component of the dipole moment perpendicular to the surface of the substrate. The glass substrate is considered to have an acidic Si-0-H groups, and 2,2'-bipyridine, the unit of PBpy, is known to coordinate acidic H to form the s-cis conformation,23a in spite of its s-trans conformation in the ordinary statee23bA model for the alignment of the PBpy molecules at the interface is depicted in Figure 9, where the PBpy molecules are coordinated to the surface Si-0-H group as depicted in Scheme 3. As described above, the vacuum-deposited PPy film is S H G inactive, which is considered to be due to the difficulty for the PPy molecule to coordinate to the Si-0-H group because of the H-T structure of PPy (vide ante). PBpy has a strong ability to coordinate with transition metals whereas PPy does not (vide infra). Treatment of the glass substrate with KOH/methanol followed by treatment with water (cf. the supplementary material) and vacuum evaporation of PBpy to this substrate leads to enhancement of the S H G intensity to about twice without loosing the anisotropy concerning the rotation around the Z axis (Figures 7 and 8a), whereas treatment of the glass substrate with dilute H2SO4 causes a decrease in the S H G intensity with loss of the anisotropy. Since the treatment with alkali is considered to increase the concentration of the Si-0-H group and that with acid to have a reverse effect, these results also support the orientation of the PBpy molecules through the coordination to the Si-0-H group. A tilt angle (0 in Figure 9b) can be roughly estimated as 30' from the anisotropic ratio shown in Figure 8a (strongest SHG intensity at 180°/weakest S H G intensity at Oo = ca. 4).24 Another interesting feature of the alignment of the PBpy molecules on the glass substrate is that the same anisotropy of the S H G is observed over all of the surface of the glass-PBpy system with an area dimension of 10 X 10 mm, indicating that the PBpy molecules are aligned in the same direction on the surface of the glass substrate as shown in Figure 9. It may be postulated that alignment of several rigidly linear PBpy molecules causes alignment of other molecules coming later. Fluorescence, Excimer-like Emission, and Other Optical Prop erties. Powdery PPy and PBpy emit strong green light when irradiated by UV light (365 and 253.7 nm). Figure 10 shows the fluorescence spectra of PPy in a dilute (2.0 X 1W mol of monomer unit dm-3) formic acid solution, in a higher concentration (0.5 mol of monomer unit dm-3) of formic acid solution, and as a film on a quartz plate, prepared by spreading a formic acid solution of PPy on the quartz plate and removing the formic acid in vacuo. Steady-state (Figure 10) and time-resolvedZ5fluorescence measurements reveal the following characteristics of the fluorescence. (a) The dilute solution (2 X 10" M) gives rise to a fluorescence with a maximum at 440 nm (Figure loa) or 2.82 eV which roughly agrees with the band gap of PPy (2.88 eV, vide ante), indicating (23) (a) Linnell, R. H.; Kaczmarczyk, A. J . Phys. Chem. 1961,65, 1196. (b) Merritt, L. L.; Schroeder, E. D. Acta Crystallogr. 1956, 9, 801. (24) Since the P-polarized light is irradiated at 45' to the surface of the PBpy film, the tilt angle 6' in Figure 9 is roughly estimated from the ratio between the maximum SHG intensity and the minimum SHG intensity of 4 (Figure Sa) according the equation cos (45 + B)/cos(45 - 6') = I/,, which gives the 6' value of about 30°. (25) Ikeda, T.; Lee, B.; Kurihara, S.;Tazuke, S.;Ito, S.;Yamamoto, M. J . Am. Chem. SOC.1988, 110,8299.
4840 J . Am. Chem. SOC.,Vol. 116, No. 11, 1994
Yamamoto et al.
t
Y
PBPY
X
A N
N
N
Glass (b) Figure 9. (a) Model for the alignment of PBpy molecules along the direction of the surface. A view of'the PBpy molecules from the Ydirection is depicted in part b. 0 is the tilt angle,
..
Vnm
Figure 10. Steady-state fluorescence spectra of PPy (a) in formic acid (2.0 X I o d M monomer unit), (b) in formic acid (0.5 M monomer unit), and (c) on a film on a quartz plate, excited at 380 nm.
Scheme 3. Coordination of PBpy Molecules to the Si-0-H Group (Upper Model, a). In the Case of PPy, Such Coordination Does Not Take Place (Lower Model, b)
--_
--
P ____
9 Y ---
s( I
(a)
PB4,
H
A --_-
1 SII
--
----
I
siI-
--
pqsl
that the fluorescence takes place by migration of electrons from the conduction band to the valence band. The energy difference between the absorption and fluorescence maxima is 2900 cm-I. An excitation spectrum of the dilute solution shows a peak a t the position of the W-P* transition (A = 373 nm). (b) When the concentration is increased to 0.5 M (Figure lob), the fluorescence spectrum exhibits a contribution from fluorescence at longer wavelength (presumably centered a t about 550 nm). A further increase in the concentration of PPy was not possible owing to limited solubility. When PPy forms a film, it gives rise to the fluorescencewithapeakat thelongerwavelength, 550nm (Figure 1Oc). PBpy film also shows a similar fluorescence with a peak at 535 nm. (c) The fluorescence lifetime measurement indicates that the fluorescence decay of the dilute PPy solution (2 X 10-6 M) is composed mainly of two components with lifetimes o f t = 60 and 250 ps when monitored at 440 nm. At the higher concentration (0.5 M), the fluorescence decay is triple-exponential with lifetimes (t) of
